As strange as this sounds is like the ability to set shows status across multiple users.
As an example the wife starts a series under her ID, that we normally watch together on our family ID. Now when I go to watch the next one on our family ID it’s not caught up. Or it shows we haven’t played.
I would live to see this on a show by show basis. As there are things we watch “at the same time” but on different episodes. (Our old favorites)
There’s an existing feature suggestion to allow “joint viewing sessions” where you basically tell Plex which users are jointly watching, so it can update the watched status for all of them. I suppose that’s what you’re actually looking for? Otherwise I don’t see why you were setting up different accounts to start with 
